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Egg (biology)
s and others (click on image for key)|right|thumb]] Orange-peel doris Acanthodoris lutea '' in tide pool laying eggs|right|thumb]] An '''egg' is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, mollusks, fish, and monotremes, an egg (Latin, ovum) is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing embryo can survive on its own. The term "egg" is restricted to the animal kingdom, similar reproductive structures in other kingdoms are termed spores, or (in spermatophytes) seeds, see ovum. Oviparous animals are animals that lay eggs, with little or no other development within the mother. Reptile eggs, bird eggs, and monotreme eggs, which are laid out of water, are surrounded by a protective shell, either flexible or inflexible. The special membranes that support these eggs are traits of all amniotes, including mammals. Eggs laid on land or in nests are usually kept within a favourable temperature range (warm) while the embryo grows. When the embryo is adequately developed it breaks out of the egg's shell. This breaking out is known as hatching. Baby animals which have just hatched are hatchlings, though standard names for babies of particular species continue to apply, such as chick for a baby chicken. Some embryos have a temporary egg tooth with which to crack, pip, or break the eggshell or covering. The ostrich egg is the largest egg currently known, though the extinct Aepyornis and some dinosaurs had larger eggs. The Bee Hummingbird produces the smallest known bird egg, which weighs half of a gram. The eggs laid by some reptiles and most fish can be even smaller, and those of insects and other invertebrates can be much smaller still. Bird eggs Bird eggs are laid by females and incubated for a time that varies according to the species; a single young hatches from each egg. Average clutch sizes range from one (as in condors) to about 17 (the Grey Partridge). Some birds lay eggs even when not fertilized (e.g. hens); it is not uncommon for pet owners to find their lone bird nesting on a clutch of unfertilized eggs, which are sometimes called wind-eggs. Colors eggs]] The default color of vertebrate eggs is the white of the calcium carbonate from which the shells are made, but some birds, mainly passerines, produce colored eggs. The pigments biliverdin and its zinc chelate give a green or blue ground color, and protoporphyrin produces reds and browns as a ground color or as spotting. Non-passerines typically have white eggs, except in some ground-nesting groups such as the Charadriiformes, sandgrouse and nightjars, where camouflage is necessary, and some parasitic cuckoos which have to match the passerine host's egg. Most passerines, in contrast, lay colored eggs, even if there is no need of cryptic colors. However some have suggested that the protoporphyrin markings on passerine eggs actually act to reduce brittleness by acting as a solid state lubricant.Solomon, S.E. (1987). Egg shell pigmentation. In Egg Quality : Current Problems and Recent Advances (eds R.G. Wells & C.G. Belyarin). Butterworths, London, pp. 147–157. If there is insufficient calcium available in the local soil, the egg shell may be thin, especially in a circle around the broad end. Protoporphyrin speckling compensates for this, and increases inversely to the amount of calcium in the soil. For the same reason, later eggs in a clutch are more spotted than early ones as the female's store of calcium is depleted. The color of individual eggs is also genetically influenced, and appears to be inherited through the mother only, suggesting that the gene responsible for pigmentation is on the sex determining W chromosome (female birds are WZ, males ZZ). It used to be thought that color was applied to the shell immediately before laying, but this research shows that coloration is an integral part of the development of the shell, with the same protein responsible for depositing calcium carbonate, or protoporphyrins when there is a lack of that mineral. In species such as the Common Guillemot, which nest in large groups,each female's eggs have very different markings, making it easier for females to identify their own eggs on the crowded cliff ledges on which they breed. Shell Bird eggshells are diverse. For example: *cormorant eggs are rough and chalky *tinamou eggs are shiny *duck eggs are oily and waterproof *cassowary eggs are heavily pitted Tiny pores in bird eggshells allow the embryo to breathe. The domestic hen's egg has around 7500 pores. Shape Most bird eggs have an oval shape, with one end rounded (the aerus) and the other more pointed (the taglion). This shape results from the egg being forced through the oviduct. Muscles contract the oviduct behind the egg, pushing it forward. The egg's wall is still shapeable, and the pointy end develops at the back. Cliff-nesting birds often have highly conical eggs. They are less likely to roll off, tending instead to roll around in a tight circle; this trait is likely to have arisen due to evolution via natural selection. In contrast, many hole-nesting birds have nearly spherical eggs. Predation Many animals feed on eggs. For example, principal predators of the Black Oystercatcher's eggs include raccoons, skunks, mink, river and sea otters, gulls, crows and foxes. The stoat (Mustela erminea) and long-tailed weasel (M. frenata) steal ducks' eggs. Snakes of the genera Dasypeltis and Elachistodon specialize in eating eggs. Brood parasitism occurs in birds when one species lays its eggs in the nest of another. In some cases, the host's eggs are removed or eaten by the female, or expelled by her chick. Brood parasites include the cowbirds and many Old World cuckoos. Various bird eggs File:Egg125o.gif|An average Whooping Crane egg is long and weighs File:Oystercatcher Eggs Norway.jpg|Eurasian oystercatcher eggs camouflaged in the nest File:Senegal egg 10s06.JPG|Egg of a Senegal Parrot, a bird that nests in tree holes, on a grid File:Comparison of eggs by Zureks.jpg|Eggs of ostrich, emu, kiwi and chicken File:Finch Egg.jpg|Finch egg next to American dime File:Ouă.jpg|Eggs of duck, goose, guineafowl and chicken File:Various_eggs.JPG|Eggs of ostrich, cassowary, chicken, flamingo, pigeon and blackbird Fish eggs Fry hatching. The larva has grown around the remains of the yolk and the remains of the soft, transparent egg are discarded.]] , in the lower right the blood vessels surround the yolk and in the upper left the black eyes are visible.]] The most common reproductive strategy for fish is known as oviparity, in which the female lays undeveloped eggs that are externally fertilized by a male. Typically large numbers of eggs are laid at one time (an adult female cod can produce 4–6 million eggs in one spawning) and the eggs are then left to develop without parental care. When the larvae hatch from the egg, they often carry the remains of the yolk in a yolk sac which continues to nourish the larvae for a few days as they learn how to swim. Once the yolk is consumed, there is a critical point after which they must learn how to hunt and feed or they will die. A few fish, notably the rays and most sharks use ovoviviparity in which the eggs are fertilized and develop internally. However the larvae still grow inside the egg consuming the egg's yolk and without any direct nourishment from the mother. The mother then gives birth to relatively mature young. In certain instances, the physically most developed offspring will devour its smaller siblings for further nutrition while still within the mother's body. This is known as intrauterine cannibalism. In certain rare scenarios, some fish such as the hammerhead shark and reef shark are viviparous, with the egg being fertilized and developed internally, but with the mother also providing direct nourishment. Other eggs Comparatively few animals give birth to live young, the vast majority have some form of eggs. Egg size and yolk Toxocara]] spawn is mesolecithal]] '' emerging fully developed from a reptilian egg]] shows the very short fetal period in marsupials.]] Vertebrate eggs can be classified by the relative amount of yolk. Simple eggs with little yolk are called microlecithal, medium sized eggs with some yolk are called mesolecithal, and large eggs with a large concentrated yolk are called macrolecithal.Romer, A. S. & Parsons, T. S. (1985): The Vertebrate Body. (6th ed.) Saunders, Philadelphia. This classification of eggs is based on the eggs of chordates, though the basic principle extends to the whole animal kingdom. Eggs with little yolk Microlecithal eggs are small with little yolk. The yolk is evenly distributed, so the cleavage of the egg cell cuts through and divides the egg into cells of fairly similar sizes. In sponges and cnidarians the dividing eggs develop directly into a simple larva, rather like a morula with cilia. In cnidarians, this stage is called the planula, and either develops directly into the adult animals or forms new adult individuals through a process of budding. Microlecithal eggs require minimal yolk mass. Such eggs are found in flatworms, roundworms, annelids, bivalves, echinoderms, the lancelet and in most marine arthropods.Barns, R.D. (1968): Invertebrate Zoology. W. B. Saunders Company, Philadelphia. 743 pages In anatomically simple animals, such as cnidarians and flatworms, the fetal development can be quite short, and even microlecithal eggs can undergo direct development. These small eggs can be produced in large numbers. In animals with high egg mortality, mirclecithal eggs are the norm, as in bivalves and marine arthropods. However, the latter are more complex anatomically than e.g. flatworms, and the small microlecithal eggs do not allow full development. Instead, the eggs hatch into larvae, which may be markedly different from the adult animal. In placental mammals, where the egg is nourished from the mother throughout the whole fetal period, the egg is reduced in size to essentially a naked egg cell (zygote). Eggs with medium yolk Mesolecithal eggs have relatively more yolk than the microlecithal eggs. The yolk is concentrated in one part of the egg (the vegetal pole), with the cell nucleus and most of the cytoplasm in the other (the animal pole). The cell cleavage is uneven, and mainly concentrated in the cytoplasma-rich animal pole.Hildebrand, M. & Gonslow, G. (2001): Analysis of Vertebrate Structure. 5th edition. John Wiley & Sons, Inc. New York The larger yolk content of the mesolecithal eggs allows for a longer fetal development. Comparatively anatomically simple animals will be able to go through the full development and leave the egg in a form reminiscent of the adult animal. This is the situation found in hagfish and some snails. Animals with smaller size eggs or more advanced anatomy will still have a distinct larval stage, though the larva will be basically similar to the adult animal, as in lampreys, coelacanth and the salamanders. Eggs with large yolk Macrolecithal eggs have a large yolk. The eggs are usually few in number, and the embryo have enough food to go through a full fetal development in most groups. Really marcrolecithal eggs are only found in selected representatives from two groups: Cephalopods and vertebrates.Nixon, M. & Messenger, J.B (eds) (1977): The Biology of Cephalopods. Symposium of the Zoological Society of London, pp 38-615 Macrolecithal eggs go through a different type of development than other eggs. Due to the large size of the yolk, the cell division can not split up the yolk mass. The fetus do in stead develop as a plate-like structure on top of the yolk mass, and only envelope it at a later stage. A portion of the yolk mass is still present as an external or semi-external yolk sac at hatching in many groups. This form of fetal development is common in bony fish, even though their eggs can be quite small. Despite their macrolecithal structure, the small size of the eggs do not allow for direct development, and the eggs hatches to a larval stage ("fry"). In terrestrial animals with macrolecithal eggs, the large volume to surface ratio necessitate structures to aid in transport of oxygen and carbon dioxide, and for storage of waste products so that the embryo do suffocate or get poisoned from its own waste while inside the egg, see amniote.Stewart J. R. (1997): Morphology and evolution of the egg of oviparous amniotes. In: S. Sumida and K. Martin (ed.) Amniote Origins-Completing the Transition to Land (1): 291-326. London: Academic Press. In addition to bony fish and cephalopods, macrolecital eggs are found in cartilaginous fish, reptiles, birds and monotreme mammals. The eggs of the coelacanths can reach a size of 9 cm in diameter, and the young go through full development while in the uterus, living of the copious yolk.Fricke, H.W. & Frahm, J. (1992): Evidence for lecithotrophic viviparity in the living coelacanth. Naturwissenschaften no 79: pp 476-479 Mammalian eggs The eggs of the egg-laying mammals (the platypus and the spiny anteaters) are macrolecithal eggs very much like those of reptiles. The eggs of marsupials are likewise macrolecithal, but rather small, and develop inside the body of the female, but do not form a placenta. The young are born at a very early stage, and can be classified as a "larva" in the biological sense.Colbert, H.E & Morales, M. (1991): Evolution of the Vertebrates - A History of Backboned Animals Through Time. 4. utgave. John Wiely & sons inc, New York. 470 pages ISBN 0-471-85074-8 In placental mammals, the egg itself is void of yolk, but develop an umbilical chord from structures that in reptiles would form the yolk sac. Receiving nutrients from the mother, the fetus complete the development while in the uterus. Overview of eggs in vertebrates Invertebrate eggs Eggs are common among invertebrates, including insects, spiders, mollusks, and crustaceans. Fish and amphibian eggs The eggs of fish and amphibians are jellylike. Fish eggs are fertilized externally, typically with the male inseminating the eggs after the female lays them. These eggs do not have a shell and would dry out in the air. Even air-breathing amphibians lay their eggs in water, or in protective foam as with the Coast foam-nest treefrog, Chiromantis xerampelina. Amniote eggs and embryos eggs in a nest dug by a female common snapping turtle (Chelydra serpentina)]] Like amphibians, amniotes are air-breathing vertebrates, but they have complex eggs or embryos, including an amniotic membrane. Amniotes include reptiles (including dinosaurs and their descendants, birds) and mammals. Reptile eggs are often rubbery and are always initially white. They are able to survive in the air. Often the sex of the developing embryo is determined by the temperature of the surroundings, with cooler temperatures favouring males. Not all reptiles lay eggs; some are viviparous ("live birth"). Among mammals, early extinct species laid eggs, as do platypuses and echidnas (spiny anteaters). Platypuses and two genera of echidna are Australian monotremes. Marsupial and placental mammals do not lay eggs, but their unborn young do have the complex tissues that identify amniotes. Human use Food Eggs laid by many different species, including birds, reptiles, amphibians, and fish, have probably been eaten by mankind for millennia. Popular choices for egg consumption are chicken, duck, roe, and caviar, but by a wide margin the egg most often humanly consumed is the chicken egg. Gallery Image:Tortoise-Hatchling.jpg|A baby tortoise emerges from its egg. Image:Emperor Gum Moth eggs2.jpg|Insect eggs, in this case those of the Emperor Gum Moth, are often laid on the underside of leaves. Image:Clupeaharenguskils2.jpg|Fish eggs, such as these herring eggs are often transparent and fertilized after laying. Image:Skate egg case (Raja binoculata) 01.jpg|Skates and some sharks have a uniquely shaped egg case called a mermaid's purse. See also *Animal shell *Bird egg *Nest building References Further reading *Andrew Gosler, Yet even more ways to dress eggs in British Birds, vol 99 no 7, July 2006 External links *Gallery of Italian bird eggs *SEM images of insect eggs Category:Eggs Category:Reproductive system Category:Oology Category:Bird breeding Category:Aviculture af:Eier ar:بيضة (أحياء) roa-rup:Gofu ast:Güevu (bioloxía) ay:K'awna bn:ডিম (জীববিজ্ঞান) zh-min-nan:Nn̄g be:Яйка bar:Oa bs:Jaje bg:Яйце (зоология) ca:Ou (biologia) cs:Vejce da:Æg de:Ei el:Αβγό eml:Óv myv:Ал es:Huevo (biología) eo:Ovo eu:Arrautza (biologia) fa:تخم (زیست‌شناسی) fr:Œuf (biologie) fy:Aai (bist) ga:Ubh gd:Ugh gl:Ovo (bioloxía) ko:알 hr:Jaje io:Amniotala ovo id:Telur ik:Mannik it:Uovo (biologia) he:ביצה jv:Endhog csb:Jôjkò ky:Жумуртка ht:Ze ku:Hêk la:Ovum lv:Ola lb:Ee lt:Kiaušinis hu:Tojás (biológia) mk:Јајце ml:മുട്ട ms:Telur (biologi) mn:Өндөг nl:Ei (dier) ja:卵 nap:Ovo no:Egg nn:Egg nrm:Oeu mhr:Муно koi:Кольть pcd:Uë pl:Jajo (biologia) pt:Ovo ro:Ou qu:Runtu ru:Яйцо sah:Сымыыт scn:Ovu (bioluggìa) simple:Egg (biology) sl:Jajce sr:Јаје sh:Jaje su:Endog fi:Muna sv:Ägg ta:முட்டை te:గుడ్డు tg:Тухм tr:Yumurta (biyoloji) uk:Яйце vec:Vóvo vi:Trứng (sinh học) zh:卵 -->